Experimental Allergic Encephalomyelitis in the Rat: Enhancement and Suppression by Synthetic Polyribonucleotides 1

Summary Lewis rats injected ip with 100 mg of rat cord homogenate developed a mild degree of experimental allergic en-cephalomyelitis (EAE). One iv injection of 0.6 mg of the polyribonucleotide complex, polyinosinic acid:polycytidylic acid (I:C), administered at the time of encephalitogenic challenge, increased the incidence and severity of EAE whereas polyadenylic acid: polyuridylic acid (A:U) and the single-stranded polyinosinic acid (I) or polycyti-dylic acid (C) had no effect. Injection of I:C at the time of onset of clinical signs of EAE did not influence the course of the illness. The mechanism of enhancement of EAE by I:C was investigated with the aid of the passive transfer system. Unexpectedly, I:C inhibited the development of EAE lesions when injected into recipients along with the fully immunized cells from donors with EAE. The inhibitory effect of I:C on EAE cells was transient and nondestructive. Inhibition by I:C was not mediated by the adrenal gland or by induction of interferon. The apparently opposite effects of I:C on active and passive production of EAE may be explained by its biphasic effects on lymphocytes and reticuloendothelial function.